@Paracosms we know that the wrapping paper is 30 inches x whatever length
I know this sounds strange - but add a square shape and press the half brick tile. Set the tile size to w - 15.5 - h 28.5 (you have to press the unlink to get the size) You can then add your image and resize to the shape
That's how I would do it (there may be other ways though )
OK, so the wrapping paper roll design area is 36" wide/long (3 feet) x 30" tall/high.
To wrap my head around what I was seeing in the design tool, I made a grid a while ago where each square represents 1" in real life. So if you count them you'll see 36 x 30 squares in both the design tool and the preview images.
For your specifics, I made a 28.5" x 15.5" rectangle in Photoshop, uploaded it to the design tool, and then with Settings--> Lock Aspect Ratio checked, positioned it in the upper right corner of the canvas and adjusted the size until it matched the red boundaries I marked for 28.5 x 15.5. Once the size is confirmed by counting the squares in the grid, you can re-position the image to whereever.
I uploaded the rectangle image at 300ppi (green), 150ppi (blue), and 72ppi (purple). I was able to re-size all of them to fit the 28.5 x 15.5 boundary, but the 72ppi gave the yellow warning error about low resolution so wasn't able to publish with that layer included. This is the live demo with the 300ppi (green) & 150ppi (blue) layers you can toggle on/off. I've lost track of where I saved the .png of just the grid to share, but you can use the demo to test that your image fits the 28.5 x 15.5 boundary without a resolution warning.
